The use of polypropylene polymers in thermoforming applications is generally known. Polypropylene polymers are typically formed into a sheet, for example, via extrusion process. The sheet is then fabricated into an article via thermoforming process. In general, the thermoforming process includes heating a sheet until it softens or starts to sag, after which one or more of vacuum, air pressure, and/or mechanical draw assist is applied and the heated sheet is drawn into a female mold, sometimes referred to as die, drawn over a male mold, or the two molds are used together to form an article, the formed article is cooled, removed from the mold, and trimmed as necessary. Thermoforming process may, for example, be employed to fabricate automobile parts. Many such automobile parts, particularly large parts, can conceivably be made by thermoforming provided the polymer has adequate processing properties such as high melt strength and end product properties such as stiffness and toughness, especially at low temperatures.
U.S. Pat. No. 6,472,473 discloses injection molded automotive articles comprising a coupled propylene polymer composition and optionally a thermoplastic elastomer, a thermoplastic polymer and a filler.
U.S. Pat. No. 6,734,253 discloses a composition comprising a coupled impact propylene copolymer. Preferably, the coupled impact propylene copolymer has an elastomeric phase that is present in an amount of at least 9 weight percent of the coupled propylene copolymer. Articles formed from the coupled propylene copolymer exhibit improved directionally balanced impact properties as compared with articles formed from comparable non-coupled impact propylene copolymers and also exhibit improved ductile-to-brittle transition temperatures compared with comparable non-coupled impact propylene copolymers.
U.S. Pat. No. 6,300,419 discloses a propylene polymer composition comprising from about 55 to about 80 parts by weight of a high crystalline isotactic propylene polymer having a crystalline phase determined by differential scanning calorimetry equal to or greater than about 62 weight percent based on the weight of the high crystalline propylene polymer; from about 20 to about 45 parts by weight of a substantially linear ethylene polymer, a linear ethylene polymer or combinations thereof wherein the substantially linear ethylene polymer and linear ethylene polymer are characterized as having a density of less than about 0.93 g/cm3, a molecular weight distribution, Mw/Mn, of less than about 3.0, and a Composition Distribution Branch Index of greater than 30 percent; from 0 to about 50 parts by weight of a filler; and from 0 to about 15 parts by weight of an additional polymer, based on the weight of the total composition.
U.S. Pat. No. 6,153,715 discloses a propylene resin composition composed of a nucleating agent and a propylene resin. Blow-molding of the propylene resin composition provides a blow-molded article which is advantageously used as, particularly, a large-sized automobile part, such as a bumper, a bumper beam, a seat back, an instrument panel, or a like part.
U.S. Pat. No. 5,414,027 discloses a normally solid, high molecular weight, non-linear, substantially gel-free, propylene polymer material, and a process for making the polymer material by high energy radiation of a normally solid, high molecular weight, linear, propylene polymer in a reduced active oxygen environment, maintaining the irradiated material in such environment for a specific period of time, and then deactivating free radicals in the material.
U.S. Patent Application Publication No. 2005/0154136 discloses a thermoplastic olefin (TPO) composition comprising (1) a major amount by weight of (a) a substantially linear homopolymer or copolymer of a C2—C10 alpha-olefin in major proportion to (b) a long chain branched a linear homopolymer or copolymer of a C2—C10 alpha-olefin, (2) a minor amount by weight of a cross linkable elastomer, and (3) at least one thermally decomposing free radical generating agent present in an amount sufficient to promote an increase in melt strength of the composition over that of the melt strength of the linear homopolymer or copolymer of a C2—C10 alpha-olefin alone and insufficient to substantially degrade the alpha-olefinic polymers. The TPO composition is prepared by melt blending the components at a temperature sufficient to melt said homopolymers or copolymers of a C2—C10 alpha-olefin and thermally decompose said agent. Articles are thermoformed from the thermoplastic olefin.
U.S. Patent Application Publication No. 2005/0070673 discloses thermoformed articles comprising a coupled propylene polymer composition.
U.S. Patent Application Publication No. 2005/0176892 discloses thermoplastic olefinic compositions comprising (a) at least one ethylene/alpha-olefin polymer having a PRR between 8 and 70 and (b) at least one polypropylene polymer and an (a)/(b) weight ratio of greater than 50/less than 50 to 90/10. The compositions are used in extrusion, calendering, blow molding, foaming and thermoforming processes to make a variety of articles, such as automotive instrument panel skins.
U.S. Patent Application Publication No. 2005/0209403 discloses thermoplastic olefinic compositions comprising (a) at least one ethylene/alpha-olefin polymer having a PRR between 8 and 70 and (b) at least one polyolefin polymer selected from the group of polyethylene homopolymers and alpha-olefin interpolymers having a PRR less than 4, ethylene/acrylic acid copolymers, ethylene/vinyl acetate copolymers, and styrenic/olefinic block interpolymers and an (a)/(b) weight ratio of greater than 50/less than 50 to 90/10. The compositions are used in extrusion, calendaring, blow molding, foaming and thermoforming processes to make a variety of articles, such as automotive instrument panel skins.
U.S. Patent Application Publication No. 2005/0272858 discloses a polypropylene resin which has a Mw/Mn of less than 5, a melt flow rate of less than 7 g/10 minutes, a 1 percent secant flexural modulus of greater than 300,000 psi and less than 2 weight percent xylene solubles.
Despite the research efforts in developing polypropylene compositions suitable for thermoforming, there is still a need for a polypropylene composition having adequate processing properties such as high melt strength and improved end product properties such as stiffness and toughness, especially at low temperatures.